Metal organic chemical vapor deposition (MOCVD) of group III-V compounds is a thin film deposition process utilizing a chemical reaction between a periodic table group III organic metal and a periodic table group V hydride. Various combinations of group III organic metal and group V hydride are possible.
This process is commonly used in the fabrication of semiconductor devices, such as light emitting diodes (LEDs). The process usually takes place in a chemical vapor deposition (CVD) reactor. CVD reactor design is a critical factor in achieving the high quality films that are required for semiconductor fabrication.
In general, the gas flow dynamics for high quality film deposition favor laminar flow. Laminar flow, as opposed to convective flow, is required to achieve high growth efficiency and uniformity. Several reactor designs are commercially available to provide laminar growth condition on a large scale, i.e., high throughput. These designs include the rotating disk reactor (RDR), the planetary rotating reactor (PRR) and the close-coupled showerhead (CCS).
However, such contemporary reactors suffer from inherent deficiencies which detract from their overall desirability, particularly with respect to high pressure and/or high temperature CVD processes. Such contemporary reactors generally work well at low pressures and relatively low temperatures (such as 30 torr and 700° C., for example). Therefore, they are generally suitable for growing GaAs, InP based compounds.
However, when growing group III nitride based compounds (such as GaN, AlN, InN, AlGaN, and InGaN), there are factors that become important when using such contemporary reactors. Unlike GaAs or InP based material, group III Nitride is preferably grown at substantially higher pressures and temperatures (generally greater that 500 torr and greater than 1000° C.). When using the aforementioned reactor designs under high pressure and temperature conditions, heavy thermal convection inherently occurs. Such thermal convection undesirably interferes with the growth process, so as to degrade efficiency and yield.
This situation worsens when the gas phase is majority ammonia. Ammonia is commonly used as nitrogen source in the III nitride MOCVD process. Ammonia is much more viscous than hydrogen. When the ambient gas contains a high percentage of ammonia, thermal convection occurs more easily than when the ambient gas is majority hydrogen, which is the case for GaAs or InP based MOCVD growth. Thermal convection is detrimental to growing high quality thin films since hard-to-control complex chemical reactions occur due to the extended duration of the presence of reactant gases in the growth chamber. This inherently results in a decrease in growth efficiency and poor film uniformity.
According to contemporary practice, a large gas flow rate is typically utilized in order to suppress undesirable thermal convection. In the growth of group III nitride, this is done by increasing the ambient gas flow rate, wherein the gas is typically a mixture of ammonia with either hydrogen or nitrogen. Therefore, high consumption of ammonia results, particularly at high growth pressure conditions. This high consumption of ammonia results in the corresponding high costs.
Reaction between source chemicals in the gas phase is another important issue in the contemporary MOCVD process for growth of GaN. This reaction also occurs in the growth of other group III-Nitrides, such as AlGaN and InGaN. Gas phase reaction is usually not desirable. However, it is not avoidable in the group III nitride MOCVD process because the reaction is severe and fast.
When the group III alkyls (such as trimethylgallium, trimethylindium, trimethylaluminum) encounter ammonia, a reaction occurs almost immediately, resulting in the undesirable formation of adducts
Usually, when these reactions occur after all the source gases enter the growth chamber, the adducts produced will participate in the actually growth process. However, if the reactions happen before or near the gas entrance of the growth chamber, the adducts produced will have an opportunity to adhere to the solid surface. If this happens, the adducts which adhere to the surface will act as gathering centers and more and more adduct will consequently tend to accumulate. This process will eventually deplete the sources, thereby making the growth process undesirably vary between runs and/or will clog the gas entrance.
An efficient reactor design for III-nitride growth does not avoid gas phase reaction, but rather controls the reaction so that it does not create such undesirable situations.
Because the demand for GaN based blue and green LEDs have increased dramatically in recent years, throughput requirements from production reactors have become important. The contemporary approach to scale up production is typically to build larger reactors. The number of wafers produced during each run has increased from 6 wafers to more than 20 wafers, while maintaining same number of runs per day, in currently available commercial reactors.
However, when a reactor is scaled up this way, several new issues arise. Because thermal convection is as severe (or even more severe) in a larger reactor as in a smaller reactor, film uniformity, as well as wafer-to-wafer uniformity, are not any better (and may be much worse). Further, at higher growth pressures, a very high gas flow rate is needed to suppress thermal convection. The amount of gas flow needed is so high that modification and special considerations are required for the gas delivery system.
Additionally, because of the high temperature requirements, the larger mechanical parts of such a scaled up (larger) reactor are inherently placed under higher thermal stress and consequently tend to break prematurely. In almost all reactor constructions, stainless steel, graphite and quartz are the most commonly used materials. Because of hydrogenation of the metals utilized (making them become brittle) and because of etching of graphite by ammonia at high temperatures, the larger metal and graphite parts tend to break down much sooner than the corresponding parts of smaller reactors. Larger quartz parts also become more susceptible to breakage because higher thermal stress.
Another issue associated with large size reactors is the difficulty in maintaining high temperature uniformity. Thickness and composition uniformity can be immediately affected by the temperature uniformity of the wafer carrier surface. In large size reactors, temperature uniformity is achieved by using a multi-zone heating configuration that is complex in design. The reliability of the heater assembly is usually poor due to the aforementioned high thermal stress and ammonia degradation. These issues of process inconsistency and extensive hardware maintenance have a significant impact on production yield and therefore product cost.
Referring now to FIG. 1, an example of a contemporary RDR reactor for use in GaN epitaxy is shown schematically. The reaction chamber has a double-walled water-cooled 10″ cylinder 11, a flow flange 12 where all the reaction or source gases are distributed and delivered into chamber 13, a rotation assembly 14 that spins the wafer carrier 16 at several hundreds of rotations per minute, a heater 17 assembly underneath the spinning wafer carrier 16 configured to heat wafers 10 to desired process temperatures, a pass through 18 to facilitate wafer carrier transfer in and out of the chamber 13, and an exhaust 19 at the center of the bottom side of the chamber 13. An externally driven spindle 21 effects rotation of the wafer carrier 16. The wafer carrier 16 comprises a plurality of pockets, each of which is configured to contain a wafer 10.
The heater 17 comprises two sets of heating elements. An inner set of heating elements 41 heats the central portion of the wafer carrier 16 and an outer set of heating elements 42 heats the periphery of the wafer carrier 16. Because the heater 17 is inside of the chamber 13, it is exposed to the detrimental effects of the reaction gases.
The spindle rotates the wafer carrier at between 500 and 1000 rpm.
As discussed previously, this design works well at lower pressures and temperatures, especially when the ambient gas is low viscosity. However, when growing GaN at high pressures and temperatures in a high ammonia ambient gas, then thermal convection occurs and gas flow tends to be undesirably turbulent.
Referring now to FIG. 2, a simplified gas streamline is shown to illustrate this turbulence. It is clear that turbulence increase as the size of the chamber and/or the distance between the wafer carrier and the top of the chamber increases. When the design of FIG. 1 is scaled up for higher throughput, the chamber 13, as well as the wafer carrier 16, is enlarged to support and contain more wafers.
Gas recirculation cells 50 tend to form when there is turbulence in the ambient gas. As those skilled in the art will appreciate, such recirculation is undesirable because it results in undesirable variations in reactant concentration and temperature. Further, such recirculation generally results in reduced growth efficiency due to ineffective use of the reactant gas.
Further, more heating zones are required in a larger reactor. This, of course, undesirably complicates the construction of such larger reactors and increases the cost thereof.
Referring now to FIGS. 3A and 3B, a comparison between a 7″ six pocket wafer carrier 16a (which supports six wafers as shown in FIG. 3A) and a 12″ twenty pocket wafer carrier 16b (which supports twenty wafers as shown in FIG. 3b) can easily be made. Each pocket 22 supports a single 2″ round wafer. From this comparison, it is clear that such scaling up of a reactor to accommodate more wafers greatly increases the size, particularly the volume, thereof. This increase in the size of the reactor results in the undesirable effects of thermal convection and the additional complexities of construction discussed above.
It is well known, however, that the depletion effect is one major drawback in contemporary horizontal reactors. As reactants in the carrier gas proceed from the center toward the peripheral of the rotating disk, a substantial amount of the reactants is consumed along the way. This undesirably makes the thin film deposited thinner and thinner along the radial direction upon the wafer.
One contemporary approach to reduce the depletion effect is to use a high gas flow rate to reduce the concentration gradient along the radial direction. The drawback of this approach is an inherent decrease in growth efficiency.
In view of the foregoing, it is desirable to provide a reactor which is not substantially susceptible to the undesirable effects of thermal convection and which can easily and economically be scaled up so as to increase throughput. It is further desirable to provide a reactor which has enhanced growth efficiency (such as by providing mixing of reactant gases immediately proximate a growth region of the wafers and by assuring intimate contact of the reactant gases with the growth region). It is yet further desirable to provide a reactor wherein the heater is outside of the chamber thereof, and is thus not exposed to the detrimental effects of the reaction gases. It is yet further desirable to mitigate the undesirable effects of depletion while maintaining growth efficiency, so as to provide enhanced deposition uniformity over the entire wafer.